Various types of grinding mills are typically employed to grind solid materials such as minerals, clay, limestone, gypsum, phosphate rock, salt, coke, biomass and coal into small particles for use in a wide range of processes such as for combustion in furnaces and for chemical reactions in reactor systems. There are many types and configurations of grinding mills including ball mills, roller mills and bowl type vertical grinding mills. The ball mills typically include a horizontal rotating cylinder containing a charge of tumbling or cascading balls. One exemplary type of roller mill is a pendulum mill which includes a support shaft rotationally supported by a bearing housing. One end of the shaft is coupled to a drive unit for rotating the shaft. An opposing end of the shaft has a hub mounted thereto. A plurality of arms extend from the hub. Each of the arms pivotally supports a roller journal which has a roller rotatingly coupled to an end thereof. The rollers rollingly engage the grinding ring. During operation of the roller mill, centrifugal forces drive the crushing members against the grinding ring. The crushing members pulverize the solid material against the grinding ring as a result of contact with the grinding ring.
As shown in FIG. 1, a roller mill 100 includes, for example, a vessel 110 in which a bowl assembly 112 is mounted. As shown in FIG. 1, the exemplary roller mill 100 include grinding rollers 118 each mounted on a suitably supported journal 120. The journals 120 are connected for rotation to a drive shaft 122 via support arms 121. The grinding rollers 118 interact with a grinding surface of the bowl assembly 112 to effect the grinding of material interposed therebetween. After being pulverized, the particles of material are thrown outwardly by centrifugal force whereby the particles of material are fed into a stream of air that is entering the mill 100 via an annular manifold 117. The flow of air is through the mill 100 is caused by a fan 119 that is in communication with a discharge duct 123 of the mill 100. The fan 119 circulates air and pulverized fine particles entrained in the air into a separator 125 (e.g., a cyclone separator or bag house) that separates the fine particles and discharges them via an outlet 125D. Circulating air that has most of the fine particles removed therefrom is discharged from the separator 125 via the clean air port 125A and circulated back to the annular manifold 117.
Prior art mills 100 typically employed a classifier 130 in a classifier section of the mill 100 located downstream of the grinding rollers 118 and upstream of the fan 119 proximate the discharge duct 123 of the mill 100. The stream of air with the particles of material entrained therein flows into a classifier 130 in which coarse particles of material are intended to be rejected from the air stream. These coarse material particles are then supposed to be returned to the grinding area for further pulverization, while the fine particles of material are supposed to be carried through the mill 100 in the air stream, and exit along with the air.
As shown in FIGS. 2 and 3 one prior art classifier 130 is known as a “whizzer separator” as disclosed in U.S. Pat. No. 2,108,609. One of the prior art classifiers 130 may be employed for the classification of the coarse particles or two or more of the prior art classifiers 130 may be employed in a series configuration. The prior art classifier 130 includes a closed central disc 138 that is secured to a rotatable shaft 130S. A plurality of blades 139 extend radially outward from the disc 138. The blades 139 are beveled inwardly and upwardly thereby defining an inclined edge 140. A conical deflector 141 is secured to a wall 130W of the classifier section of the mill 100. The conical deflector 141 defines a outwardly and downwardly sloped surface 141C. The inclined edge 140 of the blades 139 rotate in close proximity to the sloped surface 141C. There is a gap G between the sloped surface 141C and the inclined edge 140. During operation of the prior art classifier 130, air and pulverized particles entrained in the air flow through spaces 142 between adjacent blades 139. There is no flow through the central portion of the prior art separator 130 due to the presence of the disc 138.
The applicant has conducted computational fluid dynamics (CFD) analysis on the prior art classifiers 130 to determine particle velocity distributions upstream and downstream of the classifier and to determine particle size distribution. The CFD analysis demonstrated that the velocity profile of the particles upstream of the prior art classifier 130 were substantially straight and vertical with essentially no tangential velocity component or swirl. Such a velocity profile allow all sized particles, larger or small, to approach and enter the prior art classifier 130, without rejecting any of the larger particles. Thus, the separation mechanism for the prior art separator is via a “shutter effect” of the particles impinging the blades 139. For example, the substantially straight and vertical velocity profile may cause the larger particles to exit the classifier, if they hit the blades 139.
The CFD also demonstrated a strong vortex and recirculation zone downstream of the blades 139 in the prior art classifier 130. Such a vortex and recirculation zone allows a substantial amount of the small particles (e.g., including 10 micron particles) that are supposed to exit the classifier to recirculate back into the mill 100. The recirculation of the small particles back into the mill 100 reduces the efficiency and output of the prior art classifier 130.
In addition, due to the close proximity of the inclined edges 140 of the blades 139 to the sloped surface 141C of the conical deflector 141, the inclined edges 140 of the blades 139 and/or the sloped surface 141C of the conical deflector 141 tend to wear and decrease the effectiveness of the prior art classifier 130. The close proximity of the inclined edges 140 of the blades 139 to the sloped surface 141C of the conical deflector 141 creates alignment difficulties during assemble and operation. Furthermore, the prior art separator 130 is not configured to remove the heavier particles from the mill 100, but instead merely returns them to the area of the grinding rollers 118 for further grinding. This can cause operational problems with the mill as heavy and grit and hard particles such as raw sand and ground sand are maintained in the mill 100. Moreover, the prior art separator 130 cannot distinguish or separate particles based on density of the particles. As a result, the prior art separator 130 cannot distinguish or separate grit from heavy material particles suitable to be re-ground. As a result, the prior art separators discharge a mixture that contains up to about 25 weight percent undesirable materials, such as sand, grit and other larger and high density particles, with the remainder (about 75 weight percent) being the material intended to be ground. Thus, 75 percent or more of the material discharged and rejected from the roller mill 100 as waste is the useable material intended to be ground.
There is a need for an improved mill and separator system that can distinguish and separate undesirable particles from material intended to be ground and to achieve a discharge mixture that contains a higher percentage of the undesirable materials.